Chapter 3- Meiosis Flashcards
Why is meiosis known as reduction division?
Because it involved halving the chromosome number
What does the halving of chromosomes allow?
Allows a sexual life cycle with fusion of gametes
What is fertilisation?
Is the Union of sex cells, or gametes, usually from different parents
What happens just before meiosis?
DNA is replicated so that all chromosomes consist of two sister chromatids
What do the early stages of meiosis involve?
Pairing of homologous chromosomes and crossing over followed by condensation
First couple of stages of meiosis
1) homologous chromosomes pair up with each other
2) because DNA replication has already occurred, each chromosome consists of two chromatids and so there are four DNA molecules associated in each pair of homologous chromosomes
3) a pair of homologous chromosomes is bivalent and the pairing process is sometimes called synapsids
What happens after synapsis?
Process called crossing over takes place - outcome is that a junction is created where one chromatid in each homologous breaks up and rejoins win the other chromatid
What is crossing over?
Occurs random positions anywhere along the chromosomes
The orientation of homologous chromosomes is what?
Prior to separation is random which means meiosis creates genetically diverse haploid cells
What is disjunction?
Two chromosomes in each bivalent are held together by chiasmata but these slide to the end of the chromosomes and then the chromosomes can separate - one chromosome from each bivalent moves to one of the poles and the other chromosomes to the other pole
What does disjunction do?
Half the chromosomes- which means the first division is the reduction devision
What are two methods of obtaining cells for karyotype?
Chronic villus sampling and amniocentesis
What is aminocentasis?
Involves the passing of a needle through the mothers abdomen- the needle is used to withdraw a sample of amniotic fluid containing feral cells from the amniotic sac
What is chorionic villus?
A sampling tool that enters through the vagina is used to obtain cells from the chorion, one of the membranes from which the placenta developed
What is the risk of miscarriage from the 2 ways of obtaining cells a?
Aminocentasis = 1%
Chronic villus = 2%